Effects of Drought, Pest Pressure and Light Availability on Seedling Establishment and Growth: Their Role for Distribution of Tree Species across a Tropical Rainfall Gradient.

Gaviria J, Engelbrecht BM - PLoS ONE (2015)

Bottom Line:
Tree species distributions associated with rainfall are among the most prominent patterns in tropical forests.Establishment success after one year did not reflect species distribution patterns.Together these processes sort species over longer time frames, and exclude species outside their respective home range.

Affiliation: Department of Plant Ecology, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, Bayreuth, Germany.

ABSTRACTTree species distributions associated with rainfall are among the most prominent patterns in tropical forests. Understanding the mechanisms shaping these patterns is important to project impacts of global climate change on tree distributions and diversity in the tropics. Beside direct effects of water availability, additional factors co-varying with rainfall have been hypothesized to play an important role, including pest pressure and light availability. While low water availability is expected to exclude drought-intolerant wet forest species from drier forests (physiological tolerance hypothesis), high pest pressure or low light availability are hypothesized to exclude dry forest species from wetter forests (pest pressure gradient and light availability hypothesis, respectively). To test these hypotheses at the seed-to-seedling transition, the potentially most critical stage for species discrimination, we conducted a reciprocal transplant experiment combined with a pest exclosure treatment at a wet and a dry forest site in Panama with seeds of 26 species with contrasting origin. Establishment success after one year did not reflect species distribution patterns. However, in the wet forest, wet origin species had a home advantage over dry forest species through higher growth rates. At the same time, drought limited survival of wet origin species in the dry forest, supporting the physiological tolerance hypothesis. Together these processes sort species over longer time frames, and exclude species outside their respective home range. Although we found pronounced effects of pests and some effects of light availability on the seedlings, they did not corroborate the pest pressure nor light availability hypotheses at the seed-to-seedling transition. Our results underline that changes in water availability due to climate change will have direct consequences on tree regeneration and distributions along tropical rainfall gradients, while indirect effects of light and pests are less important.

pone.0143955.g001: Schematic representation of the specific hypotheses for the effects of drought, light and pests on seedling performance.For overall probability of establishment success (A) and growth (B), we hypothesized that species perform better in their respective home range outcompeting foreign species, with drought limiting wet origin species in the dry site, and pest damage limiting dry origin species in the wet site. Consequently, we expected that the pest exclosure enhances performance only for (poorly defended) dry origin species in the wet site (with high pest pressure) (i.e. three-way interaction between treatment (exclosure/control), origin (dry/wet) and site (dry/wet)). For germination (C) and survival in the wet season (D), when water availability and pest pressure are assumed to be high, we expected that wet origin species have higher survival than dry origin species in the wet site, under control conditions (site x origin interaction). We expected that (poorly defended) dry origin species are limited by pest pressure in the wet site, indicated by higher performance when pest pressure is alleviated through pest exclosure (three-way site x origin x treatment interaction). In contrast, (well defended) wet origin species exhibit no differences in germination/survival between sites, independent of the pest exclosure. We expected dry season survival (E) of (drought sensitive) wet origin species to be lower than survival of dry origin species in dry sites (significant site x origin interaction). Because pest pressure is assumed to be lower in the dry season, we expected no increase in survival with pesticide treatment for any combination of site and origin (no three-way interaction). With increasing light availability (F) we expected a stronger increase in all performance parameters for dry origin species (light x origin interaction), reflecting their higher light requirements.

Mentions:
The aim of this study was to test how the combined effects of drought, pests and light availability affect early seedling performance of tree species with contrasting origins (dry vs. wet), and how these differences in seedling performance influence species distribution patterns. We hypothesized that species have a performance advantage within their respective home (native) range compared to foreign (alien) species, resulting in exclusion of the foreign species. We expected that drought limits performance of wet forest species in drier sites (physiological tolerance hypothesis), and that pests and/or light availability limits the performance of dry forest species in wetter sites (pest pressure and light availability hypothesis, respectively). To test these hypotheses, we conducted a reciprocal transplant experiment along a rainfall gradient in Panama, with species with contrasting origins. Pests were excluded for half of the seeds, and light and soil moisture conditions were monitored during one year, including a dry and a wet season. Specific expectations for plant performance in the experiment are depicted in Fig 1.

pone.0143955.g001: Schematic representation of the specific hypotheses for the effects of drought, light and pests on seedling performance.For overall probability of establishment success (A) and growth (B), we hypothesized that species perform better in their respective home range outcompeting foreign species, with drought limiting wet origin species in the dry site, and pest damage limiting dry origin species in the wet site. Consequently, we expected that the pest exclosure enhances performance only for (poorly defended) dry origin species in the wet site (with high pest pressure) (i.e. three-way interaction between treatment (exclosure/control), origin (dry/wet) and site (dry/wet)). For germination (C) and survival in the wet season (D), when water availability and pest pressure are assumed to be high, we expected that wet origin species have higher survival than dry origin species in the wet site, under control conditions (site x origin interaction). We expected that (poorly defended) dry origin species are limited by pest pressure in the wet site, indicated by higher performance when pest pressure is alleviated through pest exclosure (three-way site x origin x treatment interaction). In contrast, (well defended) wet origin species exhibit no differences in germination/survival between sites, independent of the pest exclosure. We expected dry season survival (E) of (drought sensitive) wet origin species to be lower than survival of dry origin species in dry sites (significant site x origin interaction). Because pest pressure is assumed to be lower in the dry season, we expected no increase in survival with pesticide treatment for any combination of site and origin (no three-way interaction). With increasing light availability (F) we expected a stronger increase in all performance parameters for dry origin species (light x origin interaction), reflecting their higher light requirements.

Mentions:
The aim of this study was to test how the combined effects of drought, pests and light availability affect early seedling performance of tree species with contrasting origins (dry vs. wet), and how these differences in seedling performance influence species distribution patterns. We hypothesized that species have a performance advantage within their respective home (native) range compared to foreign (alien) species, resulting in exclusion of the foreign species. We expected that drought limits performance of wet forest species in drier sites (physiological tolerance hypothesis), and that pests and/or light availability limits the performance of dry forest species in wetter sites (pest pressure and light availability hypothesis, respectively). To test these hypotheses, we conducted a reciprocal transplant experiment along a rainfall gradient in Panama, with species with contrasting origins. Pests were excluded for half of the seeds, and light and soil moisture conditions were monitored during one year, including a dry and a wet season. Specific expectations for plant performance in the experiment are depicted in Fig 1.

Bottom Line:
Tree species distributions associated with rainfall are among the most prominent patterns in tropical forests.Establishment success after one year did not reflect species distribution patterns.Together these processes sort species over longer time frames, and exclude species outside their respective home range.

Affiliation:
Department of Plant Ecology, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, Bayreuth, Germany.

ABSTRACTTree species distributions associated with rainfall are among the most prominent patterns in tropical forests. Understanding the mechanisms shaping these patterns is important to project impacts of global climate change on tree distributions and diversity in the tropics. Beside direct effects of water availability, additional factors co-varying with rainfall have been hypothesized to play an important role, including pest pressure and light availability. While low water availability is expected to exclude drought-intolerant wet forest species from drier forests (physiological tolerance hypothesis), high pest pressure or low light availability are hypothesized to exclude dry forest species from wetter forests (pest pressure gradient and light availability hypothesis, respectively). To test these hypotheses at the seed-to-seedling transition, the potentially most critical stage for species discrimination, we conducted a reciprocal transplant experiment combined with a pest exclosure treatment at a wet and a dry forest site in Panama with seeds of 26 species with contrasting origin. Establishment success after one year did not reflect species distribution patterns. However, in the wet forest, wet origin species had a home advantage over dry forest species through higher growth rates. At the same time, drought limited survival of wet origin species in the dry forest, supporting the physiological tolerance hypothesis. Together these processes sort species over longer time frames, and exclude species outside their respective home range. Although we found pronounced effects of pests and some effects of light availability on the seedlings, they did not corroborate the pest pressure nor light availability hypotheses at the seed-to-seedling transition. Our results underline that changes in water availability due to climate change will have direct consequences on tree regeneration and distributions along tropical rainfall gradients, while indirect effects of light and pests are less important.